1. Field of the Invention
The present invention relates to a method and device configuration of a micromirror device manufactured by applying the technologies of Micro Electro Mechanical Systems (MEMS). More particularly, this invention relates to a method and device configuration for manufacturing a deflective micromirror device for preventing stiction between a micromirror and a substrate after an etching process.
2. Description of the Related Arts
After the dominance of CRT technology in the display industry, Flat Panel Display (hereafter FPD) and Projection Display gained popularity because of a smaller form-factor and a larger size of screen. In several types of projection displays, projection displays using micro-displays are gaining consumers' recognition because of high performance of image quality as well as lower cost than FPDs. There are two types of micro-displays used for projection displays in the market. One is a micro-LCD (Liquid Crystal Display) and the other is a display using micromirror technology such as a micromirror device. Because the micromirror device uses an un-polarized light, a micromirror device has an advantage of projecting images with greater brightness than the images displayed by the micro-LCD devices using the polarized light.
There are semiconductor-processing technologies that include techniques and systems for generating a micro electro mechanical structure and electric control circuits supported on a semiconductor substrate to configure the above-mentioned micromirror device. These technologies are generally referred to as MEMS (micro electro mechanical systems). Recently, the MEMS technologies have been applied in various fields such as an RF radio oscillator, an acceleration sensor, optical communications, a display, etc. In the display field, the MEMS technologies have been applied to manufacture a micromirror device as commercial products in which several millions of substantially square mirrors of about 10 μm square are arranged vertically and horizontally in a two-dimensional array.
Even though there are significant advances made on the technologies of implementing electromechanical micromirror devices as spatial light modulator in recent years, there are still limitations and difficulties when it was employed to a high quality image display. Specifically, when the display are digitally controlled, the image quality are adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror elements. In general, the number of micromirror elements required ranges from 60,000 to a several of millions in a SLM.
Referring to FIG. 1A for an image display system 1 including a screen 2 is disclosed in a reference U.S. Pat. No. 5,214,420. A light source 10 is used for generating light energy for illuminating the screen 2. The generated light 9 is further collimated and directed toward a lens 12 by a mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate light 9 into a column of light 8.
A spatial light modulator (SLM) 15 is controlled on the basis of data input by a computer 19 via a bus 18 and selectively redirects the portions of light from a path 7 toward an enlarger lens 5 and onto screen 2. The SLM 15 has a mirror array includes switchable reflective elements 17, 27, 37, and 47 each comprising a mirror 33 connected by a hinge 30 and supported on a surface 16 of a substrate in the electromechanical mirror device as shown in FIG. 1B.
When the element 17 is in one position, a portion of the light from the path 7 is redirected along a path 6 to lens 5 where it is enlarged or spread along the path 4 to impinge on the screen 2 so as to form an illuminated pixel 3. When the element 17 is in another position, the light is redirected away from the display screen 2 and hence the pixel 3 is dark.
Each of the mirror elements 17, 27, 37, and 47 implemented in a micromirror device 16 as shown in FIGS. 1A and 1B is configured to include a micromirror and an address electrode. By applying a voltage to the address electrode, the micromirror is controlled to tilt by a Coulomb force works between the micromirror and the address electrode. In this specification, the operation that causes “a micromirror tilts” is described as “a micromirror deflects”.
As the Coulomb force deflects the micromirror that also changes the direction of the reflection of incident light by the deflection angle of the mirror. In this specification, the direction of the reflected light for projecting almost all incident light toward the light path of image display is referred to as “ON light”. Conversely, as the reflected light is projected in the direction away from the light path for image display, the micromirror is referred to as operation in an “OFF state”.
A micromirror is controlled to operate in an intermediate state during the time when the micromirror is deflected in the angular positions when the incident light is reflected between the ON-state and the OFF-state. According to the system configuration of this invention, a portion of the reflected light smaller the amount of light reflected in the ON-state is controlled and directed to project a reduced light intensity for image display. The levels of gray scales are increased because the least amount of controllable light projection for image display is reduced.
By applying a voltage to the address electrode, a Coulomb force is generated to deflect the surface of the micromirror to different tilt angles and comes into contact with an address electrode or a stopper supported on the substrate. The tilt angles of the mirror surface of each of the mirror elements 17, 27, 37, and 47 are controlled to direct to different predefined angles thus allows a control circuit to control the reflection of the incident light to the ON light state or the OFF light state.
Most of the conventional image display devices such as the devices disclosed in U.S. Pat. No. 5,214,420 are implemented with a dual-state mirror control that controls the mirrors to operate at a state of either ON or OFF. The quality of an image display is limited due to the limited number of gray scales. Specifically, in a conventional control circuit that applies a PWM (Pulse Width Modulation), the quality of the image is limited by the LSB (least significant bit) or the least pulse width as control related to the ON or OFF state. Since the mirror is controlled to operate in an either ON or OFF state, the conventional image display apparatuses have no way to provide a pulse width to control the mirror that is shorter than the control duration allowable according to the LSB. The least quantity of light, which determines the least amount of adjustable brightness for adjusting the gray scale, is the light reflected during the time duration according to the least pulse width. The limited gray scale due to the LSB limitation leads to a degradation of the quality of the display image.
Specifically, FIG. 1C exemplifies a control circuit for controlling a mirror element according to the disclosure in the U.S. Pat. No. 5,285,407. The control circuit includes a memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; while transistors M6, M8, and M9 are n-channel transistors.
The capacitances C1 and C2 represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a, which is based on a Static Random Access switch Memory (SRAM) design. The transistor M9 connected to a Row-line receives a DATA signal via a Bit-line. The memory cell 32-written data is accessed when the transistor M9 that has received the ROW signal on a Word-line is turned on. The latch 32a consists of two cross-coupled inverters, i.e., M5/M6 and M7/M8, which permit two stable states, that is, a state 1 is Node A high and Node B low, and a state 2 is Node A low and Node B high.
FIG. 1D shows the “binary time periods” in the case of controlling SLM by four-bit words. As shown in FIG. 1D, the time periods have relative values of 1, 2, 4, and 8 that in turn determine the relative quantity of light of each of the four bits, where the “1” is least significant bit (LSB) and the “8” is the most significant bit. According to the PWM control mechanism, the minimum quantity of light that determines the resolution of the gray scale is a brightness controlled by using the “least significant bit” for holding the mirror at an ON position during a shortest controllable length of time.
In a simple example with n bits word for controlling the gray scale, one frame time is divided into (2n−1) time slices and each slice has an equal length of time. If one frame time is 16.7 msec., each time slice is 16.7/(2n−1) msec.
Having fixed the length for each of these time slices for each pixel of each frame, the light intensities for displaying each pixel are quantized. A pixel for displaying a black light is assigned with a 0 time slice, the intensity level represented by the LSB is 1 time slice, and maximum brightness is 2n−1 time slices. The on-time during a frame period controls the pixel's quantized intensity of display. Thus, during a frame period, the light intensity of each pixel is controlled to correspond with a quantized value of the number of time slices. The viewer's eye integrates the pixel brightness to perceive the image with a level of brightness as if the images were generated with analog levels of light.
The pulse width modulator controls the operations of addressing the deformable mirror devices by formatting the data into “bit-planes” with each bit-plane corresponding to a bit weight of the intensity value. Therefore, if each pixel's intensity is represented by an n-bit value, each frame of data has n bit-planes. Each bit-plane has a 0 or 1 value for each display element. In the PWM control process each bit plane is separately loaded during a frame and the display elements are addressed according to their associated bit-plane values. For example, the bit-plane representing the LSBs of each pixel is displayed for 1 time slice.
FIG. 2 shows the light 24 is projected from the light source into the micromirror device 16. The micromirror device 16 shown in FIG. 2 includes a plurality of mirror elements 23, and the mirror elements 23 are arranged as a two-dimensional array. Each mirror element 23 is controllable to change the tilt of the mirror surface of a micromirror 21 based on a deflection axis 22 by applying a voltage to an address electrode. Thus, in the ON light state, the micromirror 21 is controlled to tilt to the deflection angle for reflecting the incident light along the direction for projecting images onto the screen.
On the other hand, in the OFF state, the micromirror 21 is controlled to tilt to the deflection angle for reflecting the incident light projecting along the optical path toward a light dump.
In a practical implementation, the size of each micromirror 21 shown in FIG. 2 is 11 μm. However, taking into account of current trends of technological developments, it is expected that a display system would be commonly implemented with a number of pixels of 1920×1080. In the meantime, a brighter light source such as a laser light source etc. starts to replace a high-pressure lamp to project illumination light with higher light intensities. With the implementation of the laser light source, the size of a 1920×1080 micromirror array can be reduced from the current diagonal of about 0.95 inch to 0.7 inch or 0.5 inch. The size of the projection device can be further miniaturized. Furthermore, the gray scale levels for modern image display system are further increased from the current 8 bits to 10 bits and even to a higher level of 16 bits.
However, with the above-mentioned developments and trends of the micromirror device used for image display, a micromirror is still limited by the restriction that the micromirrors are frequently controlled to deflect for projecting the light according to a dual control state as the ON light state or the OFF light state.
For example, when a micromirror device is used for a TV, a micromirror deflects hundreds of billion times to satisfy a durability requirement that the TV has a life time of at least five to ten years. Under that requirement, a micromirror is expected to touch and detach a micromirror stopper hundreds of billion times. It is therefore necessary to consider the improvement of the durability of the contact part between the micromirror and the stopper.
A mirror element typically implemented in a common micromirror device has a structure of supporting a micromirror formed by a reflection layer of aluminum, silver, etc. for reflection of incident light by an elastic hinge formed by amorphous silicon, polysilicon, ceramics, aluminum, etc. on a substrate. Each micromirror further includes at least one address electrode on the substrate below the micromirror. The elastic hinge can be a horizontal hinge of a twisted spring or a vertical hinge of a bent spring. Furthermore, the substrate is made of silicon, and an electric circuit etc. connected to an address electrode is formed in the substrate. On the substrate or the address electrode, an insulating layer can be laid using the material of SiO2, Al2O3, TiN, a-Si, etc. on the substrate or the address electrode.
The micromirror device can be produced normally in a process similar to a semiconductor producing process. The manufacturing process mainly includes chemical vapor deposition (CVD), photolithography, etching, doping, chemical mechanical polishing (CMP), etc. Each mirror element of the micromirror device is controlled to deflect by a Coulomb force when a voltage is applied to the address electrode. The micromirror is controlled to deflect until it touches the insulating layer formed on the stopper or the address electrode fixed on the substrate.
The problem of stiction between the micromirror and the stopper on the substrate or the contact part of the insulating layer may occur when the micromirror contacts the stopper of the electrode. The stiction occurs due to the surface tension generated by the water content in the air condensed on the surface of the contact part on the substrate having affinity for water. Furthermore, the stiction can also occur due to the inter-molecule force, capillary force, electrostatic force, etc. between the micromirror and the contact part on the substrate, etc. The occurrence of the stiction may cause a failure of control of a micromirror.
As a countermeasure to prevent the occurrences of the stiction, different configurations and methods are applied to reduce the surface energy or decrease the area of the contact part. Alternately, the antistiction may be implemented by suppressing the friction of the contacting surfaces between the micromirrors and the stopper or electrodes.
In practical implementation, the micromirror modulator is also necessary to enclose an inactive gas, for example, argon etc. in a package for protecting a micromirror device and to guarantee the air-tightness or preventing condensation due to a change of the temperature in the operational environment. For these reasons, the antistiction configurations often become more difficult to implement.
Various countermeasures described below have been devised to prevent the above-mentioned control failures due to the occurrences of stiction.
The U.S. Pat. No. 6,815,361 aims at preventing stiction by removing a sacrifice layer during the production of a MEMS device. The patented invention discloses a method of piling an antistiction layer composed of a polymer and polycrystal and processed by dry etching with a photoresist. Then, a sacrifice layer is laid over the layer, and the antistiction layer and the sacrifice layer are simultaneously processed. The processes proceed by applying a wet etch to remove the sacrifice layer with an HF solvent and the antistiction layer is removed by dry etching.
The United states patent application Publication No. 2004/0136044 discloses a technique of performing a surface stabilizing process or providing a lube layer to reduce the stiction between a static part and a deflection part in a microstructure device having the deflection part connected to the static part.
The U.S. Pat. No. 7,057,794 discloses a micromirror device having a multiplayer structure of mirrors. In the document, there is no description of stiction.
The U.S. Pat. No. 6,114,044 discloses the configuration of a film having low surface energy on the microstructure during the process based on a liquid. Specifically low surface energy film formed by a fluorinated self-structured monolayer is provided on a microstructure device. Thus, a capillary effect generated between the components of the microstructure device as a factor of stiction, and the viscosity between the surfaces of microstructures close to each other are reduced.
The U.S. Pat. Nos. 5,602,671 and 5,411,769 disclose the technology of forming an oriented monolayer of a long-chain aliphatic halogenated polar compound including a carboxyl base (—COOH) such as PFDA (perfluorodecanoic acid; C10HF19O2) having high durability with reduced surface energy and friction coefficient to prevent the stiction caused by the force between molecules in the DMD. FIG. 3A shows a chemical formula of the PFDA. FIG. 3B is a schematic diagram of the contact part at the tips of an electrode 38 and a mirror 36 after forming an oriented monolayer of the PFDA.
The U.S. Pat. No. 5,576,878 discloses the technology of reducing the surface energy and friction using separate metals having low affinity for each other for two members contacting on a micromirror device to prevent stiction.
The United states patent application Publication No. 2004/0012061 discloses the technology of using silicide precursor, for example, a siloxane material, silane, and silanol having a completely or partially fluorinated circular structure as an antistiction material for preventing stiction.
The U.S. Pat. No. 5,447,600 discloses the technology of forming a protective layer of fluorinated polymer such as Teflon-AF (amorphous polymers) at a contact part of two members to prevent stiction on a microstructure device.
The U.S. Pat. No. 5,579,151 discloses the technology of preventing stiction by laying an inorganic layer of a solid lubricant of SiC, AIN, or SiO2 at a contact part between an electrode of a reflector and a mirror in the spatial light modulation element including a reflector capable of being electrically charged and deflecting light. Especially, it discloses the technology of laying an inorganic surface stabilizer on a static member having a thickness of about 0.5 nm to 20 nm.
The U.S. Pat. No. 6,259,551 discloses the technology of forming a monolayer film of non-volatile molecules applied between monolayers at a contact part of two members of a microstructure device.
The U.S. Pat. No. 6,576,489 discloses the technology of coating by exposing a microstructure device to alkyl silane in a gaseous phase.
The U.S. Pat. No. 6,830,950 discloses the technology of preventing stiction by forming a hydrophobic self-structured monolayer using plasma on the surface of a MEMS device. A precursor forming a self-structured monolayer can be, for example, OTS (octadecyltrichlorosilane; CH3(CH2)17SiCl3) or FDTS (perfluorodecyltrichlorosilane; CF3(CF2)7(CH2)2SiCl3). FIG. 4B shows the process of forming the FDTS of the self-structured monolayer on the surface of the MEMS device.
The U.S. Pat. No. 5,523,878 discloses the technology of covering the contact part of the microstructure device with a monomolecule using the liquid phase growth of the self-structured monolayer or the precursor. It specifically discloses an example of covering the surface of the contact part with the material of metal or aluminum oxide.
As described above, the technical difficulties of stiction are more serious in a micromirror device implemented with large number of micromirrors. There is an urgent demand to resolve the difficulties such that the image display systems implemented with micromirror devices overcome such technical problems to provide display images with improved quality.